/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package org.skfiy.util.v8;

/**
 * A {@link ForkJoinTask} with a completion action performed when triggered and
 * there are no remaining pending actions. CountedCompleters are in general more
 * robust in the presence of subtask stalls and blockage than are other forms of
 * ForkJoinTasks, but are less intuitive to program. Uses of CountedCompleter
 * are similar to those of other completion based components (such as
 * {@link java.nio.channels.CompletionHandler}) except that multiple
 * <em>pending</em> completions may be necessary to trigger the
 * {@link #onCompletion} action, not just one. Unless initialized otherwise, the
 * {@link #getPendingCount pending count} starts at zero, but may be
 * (atomically) changed using methods
 * {@link #setPendingCount}, {@link #addToPendingCount}, and {@link
 * #compareAndSetPendingCount}. Upon invocation of {@link
 * #tryComplete}, if the pending action count is nonzero, it is decremented;
 * otherwise, the completion action is performed, and if this completer itself
 * has a completer, the process is continued with its completer. As is the case
 * with related synchronization components such as
 * {@link java.util.concurrent.Phaser Phaser} and
 * {@link java.util.concurrent.Semaphore Semaphore}, these methods affect only
 * internal counts; they do not establish any further internal bookkeeping. In
 * particular, the identities of pending tasks are not maintained. As
 * illustrated below, you can create subclasses that do record some or all
 * pending tasks or their results when needed. As illustrated below, utility
 * methods supporting customization of completion traversals are also provided.
 * However, because CountedCompleters provide only basic synchronization
 * mechanisms, it may be useful to create further abstract subclasses that
 * maintain linkages, fields, and additional support methods appropriate for a
 * set of related usages.
 *
 * <p>A concrete CountedCompleter class must define method {@link
 * #compute}, that should in most cases (as illustrated below), invoke
 * {@code tryComplete()} once before returning. The class may also optionally
 * override method {@link #onCompletion} to perform an action upon normal
 * completion, and method {@link
 * #onExceptionalCompletion} to perform an action upon any exception.
 *
 * <p>CountedCompleters most often do not bear results, in which case they are
 * normally declared as {@code CountedCompleter<Void>}, and will always return
 * {@code null} as a result value. In other cases, you should override method
 * {@link #getRawResult} to provide a result from {@code join(), invoke()}, and
 * related methods. In general, this method should return the value of a field
 * (or a function of one or more fields) of the CountedCompleter object that
 * holds the result upon completion. Method {@link #setRawResult} by default
 * plays no role in CountedCompleters. It is possible, but rarely applicable, to
 * override this method to maintain other objects or fields holding result data.
 *
 * <p>A CountedCompleter that does not itself have a completer (i.e., one for
 * which {@link #getCompleter} returns {@code null}) can be used as a regular
 * ForkJoinTask with this added functionality. However, any completer that in
 * turn has another completer serves only as an internal helper for other
 * computations, so its own task status (as reported in methods such as
 * {@link ForkJoinTask#isDone}) is arbitrary; this status changes only upon
 * explicit invocations of null {@link #complete}, {@link ForkJoinTask#cancel}, {@link
 * ForkJoinTask#completeExceptionally} or upon exceptional completion of method
 * {@code compute}. Upon any exceptional completion, the exception may be
 * relayed to a task's completer (and its completer, and so on), if one exists
 * and it has not otherwise already completed. Similarly, cancelling an internal
 * CountedCompleter has only a local effect on that completer, so is not often
 * useful.
 *
 * <p><b>Sample Usages.</b>
 *
 * <p><b>Parallel recursive decomposition.</b> CountedCompleters may be arranged
 * in trees similar to those often used with {@link
 * RecursiveAction}s, although the constructions involved in setting them up
 * typically vary. Here, the completer of each task is its parent in the
 * computation tree. Even though they entail a bit more bookkeeping,
 * CountedCompleters may be better choices when applying a possibly
 * time-consuming operation (that cannot be further subdivided) to each element
 * of an array or collection; especially when the operation takes a
 * significantly different amount of time to complete for some elements than
 * others, either because of intrinsic variation (for example I/O) or auxiliary
 * effects such as garbage collection. Because CountedCompleters provide their
 * own continuations, other threads need not block waiting to perform them.
 *
 * <p>For example, here is an initial version of a class that uses divide-by-two
 * recursive decomposition to divide work into single pieces (leaf tasks). Even
 * when work is split into individual calls, tree-based techniques are usually
 * preferable to directly forking leaf tasks, because they reduce inter-thread
 * communication and improve load balancing. In the recursive case, the second
 * of each pair of subtasks to finish triggers completion of its parent (because
 * no result combination is performed, the default no-op implementation of
 * method {@code onCompletion} is not overridden). A static utility method sets
 * up the base task and invokes it (here, implicitly using the
 * {@link ForkJoinPool#commonPool()}).
 *
 * <pre> {@code
 * class MyOperation<E> { void apply(E e) { ... }  }
 *
 * class ForEach<E> extends CountedCompleter<Void> {
 *
 *   public static <E> void forEach(E[] array, MyOperation<E> op) {
 *     new ForEach<E>(null, array, op, 0, array.length).invoke();
 *   }
 *
 *   final E[] array; final MyOperation<E> op; final int lo, hi;
 *   ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
 *     super(p);
 *     this.array = array; this.op = op; this.lo = lo; this.hi = hi;
 *   }
 *
 *   public void compute() { // version 1
 *     if (hi - lo >= 2) {
 *       int mid = (lo + hi) >>> 1;
 *       setPendingCount(2); // must set pending count before fork
 *       new ForEach(this, array, op, mid, hi).fork(); // right child
 *       new ForEach(this, array, op, lo, mid).fork(); // left child
 *     }
 *     else if (hi > lo)
 *       op.apply(array[lo]);
 *     tryComplete();
 *   }
 * }}</pre>
 *
 * This design can be improved by noticing that in the recursive case, the task
 * has nothing to do after forking its right task, so can directly invoke its
 * left task before returning. (This is an analog of tail recursion removal.)
 * Also, because the task returns upon executing its left task (rather than
 * falling through to invoke {@code tryComplete}) the pending count is set to
 * one:
 *
 * <pre> {@code
 * class ForEach<E> ...
 *   public void compute() { // version 2
 *     if (hi - lo >= 2) {
 *       int mid = (lo + hi) >>> 1;
 *       setPendingCount(1); // only one pending
 *       new ForEach(this, array, op, mid, hi).fork(); // right child
 *       new ForEach(this, array, op, lo, mid).compute(); // direct invoke
 *     }
 *     else {
 *       if (hi > lo)
 *         op.apply(array[lo]);
 *       tryComplete();
 *     }
 * }
 * }</pre>
 *
 * As a further improvement, notice that the left task need not even exist.
 * Instead of creating a new one, we can iterate using the original task, and
 * add a pending count for each fork. Additionally, because no task in this tree
 * implements an {@link #onCompletion} method, {@code tryComplete()} can be
 * replaced with {@link
 * #propagateCompletion}.
 *
 * <pre> {@code
 * class ForEach<E> ...
 *   public void compute() { // version 3
 *     int l = lo,  h = hi;
 *     while (h - l >= 2) {
 *       int mid = (l + h) >>> 1;
 *       addToPendingCount(1);
 *       new ForEach(this, array, op, mid, h).fork(); // right child
 *       h = mid;
 *     }
 *     if (h > l)
 *       op.apply(array[l]);
 *     propagateCompletion();
 *   }
 * }</pre>
 *
 * Additional improvements of such classes might entail precomputing pending
 * counts so that they can be established in constructors, specializing classes
 * for leaf steps, subdividing by say, four, instead of two per iteration, and
 * using an adaptive threshold instead of always subdividing down to single
 * elements.
 *
 * <p><b>Searching.</b> A tree of CountedCompleters can search for a value or
 * property in different parts of a data structure, and report a result in an {@link
 * java.util.concurrent.atomic.AtomicReference AtomicReference} as soon as one
 * is found. The others can poll the result to avoid unnecessary work. (You
 * could additionally {@linkplain #cancel
 * cancel} other tasks, but it is usually simpler and more efficient to just let
 * them notice that the result is set and if so skip further processing.)
 * Illustrating again with an array using full partitioning (again, in practice,
 * leaf tasks will almost always process more than one element):
 *
 * <pre> {@code
 * class Searcher<E> extends CountedCompleter<E> {
 *   final E[] array; final AtomicReference<E> result; final int lo, hi;
 *   Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
 *     super(p);
 *     this.array = array; this.result = result; this.lo = lo; this.hi = hi;
 *   }
 *   public E getRawResult() { return result.get(); }
 *   public void compute() { // similar to ForEach version 3
 *     int l = lo,  h = hi;
 *     while (result.get() == null && h >= l) {
 *       if (h - l >= 2) {
 *         int mid = (l + h) >>> 1;
 *         addToPendingCount(1);
 *         new Searcher(this, array, result, mid, h).fork();
 *         h = mid;
 *       }
 *       else {
 *         E x = array[l];
 *         if (matches(x) && result.compareAndSet(null, x))
 *           quietlyCompleteRoot(); // root task is now joinable
 *         break;
 *       }
 *     }
 *     tryComplete(); // normally complete whether or not found
 *   }
 *   boolean matches(E e) { ... } // return true if found
 *
 *   public static <E> E search(E[] array) {
 *       return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
 *   }
 * }}</pre>
 *
 * In this example, as well as others in which tasks have no other effects
 * except to compareAndSet a common result, the trailing unconditional
 * invocation of {@code tryComplete} could be made conditional
 * ({@code if (result.get() == null) tryComplete();}) because no further
 * bookkeeping is required to manage completions once the root task completes.
 *
 * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine results of
 * multiple subtasks usually need to access these results in method
 * {@link #onCompletion}. As illustrated in the following class (that performs a
 * simplified form of map-reduce where mappings and reductions are all of type
 * {@code E}), one way to do this in divide and conquer designs is to have each
 * subtask record its sibling, so that it can be accessed in method
 * {@code onCompletion}. This technique applies to reductions in which the order
 * of combining left and right results does not matter; ordered reductions
 * require explicit left/right designations. Variants of other streamlinings
 * seen in the above examples may also apply.
 *
 * <pre> {@code
 * class MyMapper<E> { E apply(E v) {  ...  } }
 * class MyReducer<E> { E apply(E x, E y) {  ...  } }
 * class MapReducer<E> extends CountedCompleter<E> {
 *   final E[] array; final MyMapper<E> mapper;
 *   final MyReducer<E> reducer; final int lo, hi;
 *   MapReducer<E> sibling;
 *   E result;
 *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
 *              MyReducer<E> reducer, int lo, int hi) {
 *     super(p);
 *     this.array = array; this.mapper = mapper;
 *     this.reducer = reducer; this.lo = lo; this.hi = hi;
 *   }
 *   public void compute() {
 *     if (hi - lo >= 2) {
 *       int mid = (lo + hi) >>> 1;
 *       MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
 *       MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
 *       left.sibling = right;
 *       right.sibling = left;
 *       setPendingCount(1); // only right is pending
 *       right.fork();
 *       left.compute();     // directly execute left
 *     }
 *     else {
 *       if (hi > lo)
 *           result = mapper.apply(array[lo]);
 *       tryComplete();
 *     }
 *   }
 *   public void onCompletion(CountedCompleter<?> caller) {
 *     if (caller != this) {
 *       MapReducer<E> child = (MapReducer<E>)caller;
 *       MapReducer<E> sib = child.sibling;
 *       if (sib == null || sib.result == null)
 *         result = child.result;
 *       else
 *         result = reducer.apply(child.result, sib.result);
 *     }
 *   }
 *   public E getRawResult() { return result; }
 *
 *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
 *     return new MapReducer<E>(null, array, mapper, reducer,
 *                              0, array.length).invoke();
 *   }
 * }}</pre>
 *
 * Here, method {@code onCompletion} takes a form common to many completion
 * designs that combine results. This callback-style method is triggered once
 * per task, in either of the two different contexts in which the pending count
 * is, or becomes, zero: (1) by a task itself, if its pending count is zero upon
 * invocation of {@code
 * tryComplete}, or (2) by any of its subtasks when they complete and decrement
 * the pending count to zero. The {@code caller} argument distinguishes cases.
 * Most often, when the caller is {@code this}, no action is necessary.
 * Otherwise the caller argument can be used (usually via a cast) to supply a
 * value (and/or links to other values) to be combined. Assuming proper use of
 * pending counts, the actions inside {@code onCompletion} occur (once) upon
 * completion of a task and its subtasks. No additional synchronization is
 * required within this method to ensure thread safety of accesses to fields of
 * this task or other completed tasks.
 *
 * <p><b>Completion Traversals</b>. If using {@code onCompletion} to process
 * completions is inapplicable or inconvenient, you can use methods
 * {@link #firstComplete} and {@link #nextComplete} to create custom traversals.
 * For example, to define a MapReducer that only splits out right-hand tasks in
 * the form of the third ForEach example, the completions must cooperatively
 * reduce along unexhausted subtask links, which can be done as follows:
 *
 * <pre> {@code
 * class MapReducer<E> extends CountedCompleter<E> { // version 2
 *   final E[] array; final MyMapper<E> mapper;
 *   final MyReducer<E> reducer; final int lo, hi;
 *   MapReducer<E> forks, next; // record subtask forks in list
 *   E result;
 *   MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
 *              MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
 *     super(p);
 *     this.array = array; this.mapper = mapper;
 *     this.reducer = reducer; this.lo = lo; this.hi = hi;
 *     this.next = next;
 *   }
 *   public void compute() {
 *     int l = lo,  h = hi;
 *     while (h - l >= 2) {
 *       int mid = (l + h) >>> 1;
 *       addToPendingCount(1);
 *       (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork;
 *       h = mid;
 *     }
 *     if (h > l)
 *       result = mapper.apply(array[l]);
 *     // process completions by reducing along and advancing subtask links
 *     for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
 *       for (MapReducer t = (MapReducer)c, s = t.forks;  s != null; s = t.forks = s.next)
 *         t.result = reducer.apply(t.result, s.result);
 *     }
 *   }
 *   public E getRawResult() { return result; }
 *
 *   public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
 *     return new MapReducer<E>(null, array, mapper, reducer,
 *                              0, array.length, null).invoke();
 *   }
 * }}</pre>
 *
 * <p><b>Triggers.</b> Some CountedCompleters are themselves never forked, but
 * instead serve as bits of plumbing in other designs; including those in which
 * the completion of one of more async tasks triggers another async task. For
 * example:
 *
 * <pre> {@code
 * class HeaderBuilder extends CountedCompleter<...> { ... }
 * class BodyBuilder extends CountedCompleter<...> { ... }
 * class PacketSender extends CountedCompleter<...> {
 *   PacketSender(...) { super(null, 1); ... } // trigger on second completion
 *   public void compute() { } // never called
 *   public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
 * }
 * // sample use:
 * PacketSender p = new PacketSender();
 * new HeaderBuilder(p, ...).fork();
 * new BodyBuilder(p, ...).fork();
 * }</pre>
 *
 * @since 1.8
 * @author Doug Lea
 */
public abstract class CountedCompleter<T> extends ForkJoinTask<T> {

    private static final long serialVersionUID = 5232453752276485070L;
    /**
     * This task's completer, or null if none
     */
    final CountedCompleter<?> completer;
    /**
     * The number of pending tasks until completion
     */
    volatile int pending;

    /**
     * Creates a new CountedCompleter with the given completer and initial
     * pending count.
     *
     * @param completer this task's completer, or {@code null} if none
     * @param initialPendingCount the initial pending count
     */
    protected CountedCompleter(CountedCompleter<?> completer,
            int initialPendingCount) {
        this.completer = completer;
        this.pending = initialPendingCount;
    }

    /**
     * Creates a new CountedCompleter with the given completer and an initial
     * pending count of zero.
     *
     * @param completer this task's completer, or {@code null} if none
     */
    protected CountedCompleter(CountedCompleter<?> completer) {
        this.completer = completer;
    }

    /**
     * Creates a new CountedCompleter with no completer and an initial pending
     * count of zero.
     */
    protected CountedCompleter() {
        this.completer = null;
    }

    /**
     * The main computation performed by this task.
     */
    public abstract void compute();

    /**
     * Performs an action when method {@link #tryComplete} is invoked and the
     * pending count is zero, or when the unconditional method {@link #complete}
     * is invoked. By default, this method does nothing. You can distinguish
     * cases by checking the identity of the given caller argument. If not equal
     * to {@code
     * this}, then it is typically a subtask that may contain results (and/or
     * links to other results) to combine.
     *
     * @param caller the task invoking this method (which may be this task
     * itself)
     */
    public void onCompletion(CountedCompleter<?> caller) {
    }

    /**
     * Performs an action when method {@link #completeExceptionally} is invoked
     * or method {@link #compute} throws an exception, and this task has not
     * otherwise already completed normally. On entry to this method, this task {@link
     * ForkJoinTask#isCompletedAbnormally}. The return value of this method
     * controls further propagation: If {@code true} and this task has a
     * completer, then this completer is also completed exceptionally. The
     * default implementation of this method does nothing except return
     * {@code true}.
     *
     * @param ex the exception
     * @param caller the task invoking this method (which may be this task
     * itself)
     * @return true if this exception should be propagated to this task's
     * completer, if one exists
     */
    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
        return true;
    }

    /**
     * Returns the completer established in this task's constructor, or
     * {@code null} if none.
     *
     * @return the completer
     */
    public final CountedCompleter<?> getCompleter() {
        return completer;
    }

    /**
     * Returns the current pending count.
     *
     * @return the current pending count
     */
    public final int getPendingCount() {
        return pending;
    }

    /**
     * Sets the pending count to the given value.
     *
     * @param count the count
     */
    public final void setPendingCount(int count) {
        pending = count;
    }

    /**
     * Adds (atomically) the given value to the pending count.
     *
     * @param delta the value to add
     */
    public final void addToPendingCount(int delta) {
        int c; // note: can replace with intrinsic in jdk8
        do {
        } while (!U.compareAndSwapInt(this, PENDING, c = pending, c + delta));
    }

    /**
     * Sets (atomically) the pending count to the given count only if it
     * currently holds the given expected value.
     *
     * @param expected the expected value
     * @param count the new value
     * @return true if successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return U.compareAndSwapInt(this, PENDING, expected, count);
    }

    /**
     * If the pending count is nonzero, (atomically) decrements it.
     *
     * @return the initial (undecremented) pending count holding on entry to
     * this method
     */
    public final int decrementPendingCountUnlessZero() {
        int c;
        do {
        } while ((c = pending) != 0
                && !U.compareAndSwapInt(this, PENDING, c, c - 1));
        return c;
    }

    /**
     * Returns the root of the current computation; i.e., this task if it has no
     * completer, else its completer's root.
     *
     * @return the root of the current computation
     */
    public final CountedCompleter<?> getRoot() {
        CountedCompleter<?> a = this, p;
        while ((p = a.completer) != null) {
            a = p;
        }
        return a;
    }

    /**
     * If the pending count is nonzero, decrements the count; otherwise invokes
     * {@link #onCompletion} and then similarly tries to complete this task's
     * completer, if one exists, else marks this task as complete.
     */
    public final void tryComplete() {
        CountedCompleter<?> a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                a.onCompletion(s);
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            } else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) {
                return;
            }
        }
    }

    /**
     * Equivalent to {@link #tryComplete} but does not invoke {@link
     * #onCompletion} along the completion path: If the pending count is
     * nonzero, decrements the count; otherwise, similarly tries to complete
     * this task's completer, if one exists, else marks this task as complete.
     * This method may be useful in cases where {@code onCompletion} should not,
     * or need not, be invoked for each completer in a computation.
     */
    public final void propagateCompletion() {
        CountedCompleter<?> a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            } else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) {
                return;
            }
        }
    }

    /**
     * Regardless of pending count, invokes {@link #onCompletion}, marks this
     * task as complete and further triggers {@link
     * #tryComplete} on this task's completer, if one exists. The given
     * rawResult is used as an argument to {@link #setRawResult} before invoking
     * {@link #onCompletion} or marking this task as complete; its value is
     * meaningful only for classes overriding {@code setRawResult}.
     *
     * <p>This method may be useful when forcing completion as soon as any one
     * (versus all) of several subtask results are obtained. However, in the
     * common (and recommended) case in which {@code
     * setRawResult} is not overridden, this effect can be obtained more simply
     * using {@code quietlyCompleteRoot();}.
     *
     * @param rawResult the raw result
     */
    public void complete(T rawResult) {
        CountedCompleter<?> p;
        setRawResult(rawResult);
        onCompletion(this);
        quietlyComplete();
        if ((p = completer) != null) {
            p.tryComplete();
        }
    }

    /**
     * If this task's pending count is zero, returns this task; otherwise
     * decrements its pending count and returns {@code
     * null}. This method is designed to be used with {@link
     * #nextComplete} in completion traversal loops.
     *
     * @return this task, if pending count was zero, else {@code null}
     */
    public final CountedCompleter<?> firstComplete() {
        for (int c;;) {
            if ((c = pending) == 0) {
                return this;
            } else if (U.compareAndSwapInt(this, PENDING, c, c - 1)) {
                return null;
            }
        }
    }

    /**
     * If this task does not have a completer, invokes {@link
     * ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if this
     * task's pending count is non-zero, decrements its pending count and
     * returns {@code null}. Otherwise, returns the completer. This method can
     * be used as part of a completion traversal loop for homogeneous task
     * hierarchies:
     *
     * <pre> {@code
     * for (CountedCompleter<?> c = firstComplete();
     *      c != null;
     *      c = c.nextComplete()) {
     *   // ... process c ...
     * }}</pre>
     *
     * @return the completer, or {@code null} if none
     */
    public final CountedCompleter<?> nextComplete() {
        CountedCompleter<?> p;
        if ((p = completer) != null) {
            return p.firstComplete();
        } else {
            quietlyComplete();
            return null;
        }
    }

    /**
     * Equivalent to {@code getRoot().quietlyComplete()}.
     */
    public final void quietlyCompleteRoot() {
        for (CountedCompleter<?> a = this, p;;) {
            if ((p = a.completer) == null) {
                a.quietlyComplete();
                return;
            }
            a = p;
        }
    }

    /**
     * Supports ForkJoinTask exception propagation.
     */
    void internalPropagateException(Throwable ex) {
        CountedCompleter<?> a = this, s = a;
        while (a.onExceptionalCompletion(ex, s)
                && (a = (s = a).completer) != null && a.status >= 0) {
            a.recordExceptionalCompletion(ex);
        }
    }

    /**
     * Implements execution conventions for CountedCompleters.
     */
    protected final boolean exec() {
        compute();
        return false;
    }

    /**
     * Returns the result of the computation. By default returns {@code null},
     * which is appropriate for {@code Void} actions, but in other cases should
     * be overridden, almost always to return a field or function of a field
     * that holds the result upon completion.
     *
     * @return the result of the computation
     */
    public T getRawResult() {
        return null;
    }

    /**
     * A method that result-bearing CountedCompleters may optionally use to help
     * maintain result data. By default, does nothing. Overrides are not
     * recommended. However, if this method is overridden to update existing
     * objects or fields, then it must in general be defined to be thread-safe.
     */
    protected void setRawResult(T t) {
    }
    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long PENDING;

    static {
        try {
            U = getUnsafe();
            PENDING = U.objectFieldOffset(CountedCompleter.class.getDeclaredField("pending"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating into a
     * jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException tryReflectionInstead) {
        }
        try {
            return java.security.AccessController.doPrivileged(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
                public sun.misc.Unsafe run() throws Exception {
                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
                        f.setAccessible(true);
                        Object x = f.get(null);
                        if (k.isInstance(x)) {
                            return k.cast(x);
                        }
                    }
                    throw new NoSuchFieldError("the Unsafe");
                }
            });
        } catch (java.security.PrivilegedActionException e) {
            throw new RuntimeException("Could not initialize intrinsics",
                    e.getCause());
        }
    }
}
